The invention relates generally to the display of digital images. More specifically, the invention relates to an apparatus and method of performing dithering of color images.
A computer monitor having 24-bit color capability can display photorealistic images. Such high quality can be achieved in part because the 24-bit color capability allows the computer monitor a choice of 16.7 million discrete colors for each individual pixel of an image being displayed.
A conventional inkjet printer, on the other hand, does not have a choice of 16.7 million colors for printing an image. The conventional inkjet printer deposits dots of colored ink on a sheet of paper. Within the space of each dot, the conventional inkjet printer might deposit yellow, cyan and magenta ink in combinations that produce one of eight solid colors: cyan, yellow, magenta, black, white, red, green and blue. Thus, the conventional inkjet printer has a choice of eight printable colors at each pixel.
The inkjet printer might use a digital halftoning method to create a perception that it is printing with a wider range of colors than it actually is. When printing a color patch, the printer can place dots of different colors in a high frequency pattern. When viewing the high frequency pattern, the human visual system xe2x80x9cappliesxe2x80x9d a low-pass filter and perceives only the average color of the dots in the pattern. Thus, a dot pattern including black and white dots will appear as a shade of gray. The shade of gray will depend upon the relative quantities of black and white dots in the pattern.
Monochrome dithering is a halftoning method in which the gray values of a monochrome image are compared against an array of thresholds. Ink dots are placed at those locations where the pixel values are less than or equal to the corresponding threshold values. Standard (Cartesian) color dithering is an overlay of three monochrome dithering processes, each of which is applied separately to a single color plane. When a patch of a halftone color is printed, potentially all eight colors are used.
To produce a good color halftone, the color dots are placed such that their placement pattern is visually unnoticeable. Additionally, the dot colors are selected such that the local average color of the dots matches the desired color of the color patch.
However, there are problems associated with halftoning methods, especially at printing resolutions of up to 600 dots per inch (xe2x80x9cdpixe2x80x9d). Because inkjet dots at 600 dpi can still be resolved by the human eye, the dots might be perceived as artifacts and other types of noise. The noise can degrade the quality of the printed image. A grainy color image might result.
Efforts have been made to improve print quality by increasing the color choices at each dot. The color choices have been increased by controlling the amount of ink deposited at each dot. Varying the amount of ink can control the intensity at each dot. The HP DeskJet 850C inkjet printer, available from the Hewlett-Packard Company, can vary the amount of ink deposited at each dot to print up to 64 different color and grayscales choices at each dot. Smoother dither patterns and halftones result in a less grainy image.
Other efforts have been made to minimize the noise in the image in order to produce smoother dither patterns and halftones. For example, efforts have been directed towards determining optimal patterns for placing the dots. Efforts have also been directed towards determining an optimal number of dots that should be used for each color.
Efforts have also been directed towards reducing the contrast in color combinations used to render color patches. The color contrast may be minimized by minimizing the number of black and white dots used in the same color patch. Rather than printing black dots on a white background, grays using non-overlapping cyan, magenta and yellow dots would be printed.
Contrast in color combinations used to render color patches may be reduced by the present invention. A plurality of thresholds are applied to a Cyan color plane of a color image, and an inverse of the plurality of thresholds is applied to a Magenta color plane of the color image. A Yellow color plane of the color image is also processed. In this manner, the color image is converted into a color pattern for display by an imaging device such as a printer.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.